The invention relates to a capacitive pressure sensor.
Such pressure sensors are frequently used in process automation in order to measure the pressure of different process media, which can be present as liquids, gases or vapors.
Such pressure sensors essentially comprise a basic body and a diaphragm, which both preferably consist of a ceramic or a monocrystalline material. Provided on the basic body is a flat cutout which is also called a diaphragm bed and which is completely covered by the diaphragm.
The diaphragm bed and the diaphragm delimit a measuring chamber which is separated from the actual process medium and which is generally filled with air or with a silicon oil as hydraulic medium. The pressure chamber is gas tight or liquid tight. This requires a substantial outlay in the production of the connection between the diaphragm and basic body.
Provided in each case on the diaphragm bed and the underside, facing the diaphragm bed, of the diaphragm are electrodes which are mostly applied using sputtering technology, vapor deposition methods or, for example, the screen printing method, as described in U.S. Pat. No. 5,050,035, for example. These two electrodes together form the actual measuring capacitor whose measuring signal is evaluated.
If a reference pressure PR acts on the diaphragm, and if this pressure differs from the pressure prevailing in the pressure chamber, the diaphragm is deformed elastically. This leads to a change in the spacing of the two electrodes, and thus to a change in capacitance of the measuring capacitor. The capacitance of the measuring capacitor is a measure of the pressure difference. It is detected as a measuring signal with an electronic evaluation system to which two electrodes are connected, and is evaluated.
A distinction is made between pressure sensors for relative pressure, absolute pressure and differential pressure, depending on which reference pressure is present in the pressure chamber or on the outside of the diaphragm.
It is customary to speak only of the pressure which is measured, and not of the pressure difference, as would actually be appropriate.
In addition to simple pressure sensors, so-called differential pressure sensors are also known which detect the difference between two process pressures. Such differential pressure sensors consist, for example, of two such described pressure sensors, with the difference that they have a common basic body. The measuring chambers are located on the opposite sides of the basic body. They are interconnected by a connecting channel which serves the purpose of pressure compensation.
In the case of a further differential pressure sensor, two measuring chambers are separated from one another in a basic body by a common diaphragm.
In both cases, the pressure difference between the process pressures prevailing on the two sides of the basic body is the measured variable of interest.
The diaphragm and basic body are interconnected via a joint. In the case of a ceramic sensor, the joining can be performed by means of active solder or a glass frit. It is possible in the case of a sensor made from monocrystalline material to make use, for example, of eutectic bonding, anodic bonding or fusion bonding as the joining technique.
The diaphragm, basic body and the joint itself are very strongly loaded at the joint by a stress concentration as a consequence of notch stresses when a high pressure prevails in one of the measuring chambers or also in both measuring chambers. In the extreme case, cracks can form in the diaphragm or in the basic body or the connection between the diaphragm and basic body can be torn apart, and this leads to a failure of the pressure sensor.
U.S. Pat. No. 5,520,054 discloses a pressure sensor in the case of which the wall in the region of the joint is widened in order to reduce the loading of the joint. This measure is very complicated in terms of production technology. Moreover, there is a reduction in the stiffness of the regions of the ceramic which border on the joint. As a result, it is only the stress directly at the joint which is reduced. The stress maximum continues nevertheless to be located in the region of the joint.
It is the object of the invention to create a pressure sensor which displaces into the basic body the stress concentrations produced as a consequence of notch stresses at the root of the joint, since the connection between the diaphragm and basic body is mostly weaker than the bulk material of the basic body. A further object of the invention is not only to displace the site of the stress concentration, but to reduce the maximum stresses. It is also to be possible for the pressure sensor to be produced simply and cost effectively.
This object is achieved by means of a pressure sensor having a basic body, a diaphragm connected to the basic body via a joint, a measuring capacitor for generating a measuring signal with a first and second electrode, which are respectively applied opposite one another on the diaphragm and on the basic body, a groove adjoining in the basic body at the end of the root of the joint.
The groove reduces stress concentrations in the region of the joint.
Advantageous developments of the invention are specified in the subclaims.
The following discussion applies to capacitive pressure sensors and capacitive differential pressure sensors correspondingly, and so for the sake of simplicity only capacitive pressure sensors will be treated.